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I have a DMX controller using a 3-pin connector (gnd, data+ and data-). I need to connect it to some cheap chinese DMX lights.

The problem is that the lights are powered over the same 3 lines using a 24 VDC power supply. When I did, it burned out my controller. DMX is a differential signal, so it might work, but 24 V is just too much (and clearly out of specs of DMX standard, which allow for at most 7 V). These are the lights so there would be no problem with other DMX lights.

The 24 DC power is connected to gnd (pin 1) and data+ (pin 3) of the DMX lights and is needed to power the lights.

As extra info: my DMX controller is running on the power of a USB port. When using a multimeter in AC mode (with the lights and 24 V disconnected) it measures 2.5-3 V on the data channel, while it is sending out some data.

How to prevent my new (expensive) controller to burn out as well (it shouldn't see the 24 V)?

schematic

simulate this circuit – Schematic created using CircuitLab

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  • \$\begingroup\$ I just found an optocoupler that could stand 24V (max) toshiba.semicon-storage.com/info/…. That might just work. Question than becomes: how to connect it? \$\endgroup\$
    – illu
    Aug 15, 2019 at 19:48
  • \$\begingroup\$ Hello illu, have you considered something like the EntTec D-Split? I'm not 100% sure this will work for you but thought I'd share. We don't normally give product recommendations either, but here it is in case no viable solution can be found. Of course the best solution would be to get some lights that do not violate the DMX spec this grossly. \$\endgroup\$
    – rdtsc
    Aug 16, 2019 at 0:44
  • \$\begingroup\$ I just mailed them the question if d-split can handle 24V on its output. I will be using 640 of these lights, Adding a few extra bucks per light quickly adds up in total costs. I only have 16 at the moment for testing, so switching to a different type is an option, but I couldn't find anything affordable. \$\endgroup\$
    – illu
    Aug 16, 2019 at 8:09

4 Answers 4

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Devices like the LTC1785 RS485 transceiver can tolerate up to 60V on the bus lines. Replace the 75176 or MAX485 chips with the LTC1785 and the problem is solved.

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  • \$\begingroup\$ that sounds like a good idea. I'll look into the details. Could I just add an ltc1785 after the existing dmx unit without modifying its electronics? I guess so. \$\endgroup\$
    – illu
    May 26, 2021 at 10:31
  • \$\begingroup\$ 'just add' sounds so easy, but do you understand what is involved? If you're lacking the skills, find someone who does have the skills or use an 'off the shelf' solution. \$\endgroup\$
    – Kartman
    May 26, 2021 at 10:57
  • \$\begingroup\$ looking from the specs I should connect both DE (driver output enable) and RE (not receiver enabled) to a high value, and and perhaps take one line of the dmx and connect that to DI (data input). Then use the A,B, and GND as the wires to transmit the dmx signal. \$\endgroup\$
    – illu
    May 26, 2021 at 12:06
  • \$\begingroup\$ See if that works for you. You might want to add a bypass capacitor and some 10 Ohm series resistors on the A & B \$\endgroup\$
    – Kartman
    May 26, 2021 at 12:11
  • \$\begingroup\$ thanks I'll do that. just ordered two ltc1785 \$\endgroup\$
    – illu
    May 26, 2021 at 12:48
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One option could be to insert a capacitor between the dmx controller data+ pin3 terminal and the signal wire. A capacitor filters out the 24V DC supply completely so that it can not harm the controller. At the same time, the AC signal should pass the capacitor. (To learn more about this, ask google for "first order RC high pass filter", e.g. here).

But it would be a bit of engineering to get the value of that capacitor right. It depends on the bit rate and also the impedances of the controller and receiver. You might want to start with a 1uF capacitor rated for 50V.

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  • \$\begingroup\$ I think DMX signal is 250kbit/sec, does that help in determining the value of the capacitor? \$\endgroup\$
    – illu
    Aug 15, 2019 at 13:48
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    \$\begingroup\$ See my answer. This will not work because DMX isn’t DC-balanced. \$\endgroup\$
    – hacktastical
    Aug 15, 2019 at 15:00
  • \$\begingroup\$ It would be very easy to check if it works with a 1u capacitor or not. Why don‘t you just give it a try? \$\endgroup\$
    – Stefan Wyss
    Aug 16, 2019 at 10:36
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It sounds like the system is designed to be used with a power supply for XLR phantom power, which is normally used for certain types of microphone. A standalone phantom power supply will both provide the power and isolate the output from that power, so that should be sufficient to protect your controller.

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  • \$\begingroup\$ my controller is battery powered when running in stand-alone mode (from a usb battery pack). the leds require a huge amount of power. The power supply is rated at 24V/10A. Base on your answer I searched for phantom power supplies. These deliver only a tiny amount of power. Perhaps some of the circuits used for phantom power might be useful, but I don't know how exactly. \$\endgroup\$
    – illu
    Aug 15, 2019 at 14:25
  • \$\begingroup\$ Phantom power is typically 48v here in the 'states for pro gear. There is a long and convoluted history of "phantom power" dating back to the invention of the telephone. Consumer gadgets and other oddities "phantom power" today can be almost anything: 3v, 10v, 12v, etc. \$\endgroup\$
    – rdtsc
    Aug 16, 2019 at 0:39
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Optical isolation could be used on the TX (controller) side to allow the DMX pair to carry power.

Unfortunately any kind of AC coupling isn’t going to work with DMX as it isn’t designed to maintain DC balance. So no transformers or caps for example.

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  • \$\begingroup\$ so how to do this optical isolation? I did search for devices that do that, but the ones I found only allow a max of 7 or 12 Volt. \$\endgroup\$
    – illu
    Aug 15, 2019 at 15:11
  • \$\begingroup\$ You would need to do something like make a voltage divider between GND and D+ / 24V and wire the isolator transistor across that. \$\endgroup\$
    – hacktastical
    Aug 15, 2019 at 15:18
  • \$\begingroup\$ That sounds like a plan, but how? my electronic skills are pretty low. I know how to solder, I know what a transistor is, but lack the knowledge to design a circuit with components. \$\endgroup\$
    – illu
    Aug 15, 2019 at 15:30
  • \$\begingroup\$ If you have some info about how the light’s DMX receiver works that would be helpful. \$\endgroup\$
    – hacktastical
    Aug 15, 2019 at 15:33
  • \$\begingroup\$ Link to a datasheet for the light for instance. \$\endgroup\$
    – hacktastical
    Aug 15, 2019 at 15:56

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